The secondary fluid, usually water, contains particles of matter, principally in the form of iron oxides or copper compounds, or alternatively traces of other metals, which tend to deposit on the tube plate, in particular in the central region thereof, between the closest branches of the U-shaped tubes where the speed of flow of the secondary fluid from one end of the plate to the other is insufficient to prevent deposition of sludge or other residues, which are harmful because they create concentrations of corrosive agents along the outer walls of these tubes.
The means of blockage or partial occupation arranged in this central region, generally called the "tube lane", by reference to the term of art "tube lane blocking device", the term blocking in fact rather indicating obstruction of this region, reduce to a minimum the flow of the secondary fluid in the corresponding passage between the closest tubes, with a view to increasing the lateral flow through the bundle, while reducing the deposition of sludge in this passage. These means generally consist of parallelepipedal metal blocks, mounted permanently in the central region between the tubes and resting on the plate along the diameter thereof in this region.
It should be noted that, depending on whether the steam generator is of the "axial economizer steam generator" or "boiler steam generator"type, the tube lane constituting the aforementioned central region may or may not be separated in the mid-plane of the generator by a vertical deflector, fixed to the tube plate and extending at the middle of the passage delimited between the branches of the tubes in this region. In the first case, the blocking means are in a single piece and housed in the latter. In the second case, they are formed of two symmetrical parts, arranged respectively on either side of the vertical deflector, between the deflector and the branches of the facing tubes.
With such blocking means which are immobilized on the tube plate, it is sometimes difficult to carry out the necessary maintenance, in particular in the tube plate surface which points towards the inside of the steam generator, for cleaning the tube plate or for checking the tubes and especially their linkage with the plate. Furthermore, when the tube plate is equipped with a continuous blow-down device, including in particular a pipeline parallel to the plate and pierced with holes distributed over its length for continuous sampling of water through the plate, the presence of these means may hinder optimal operation.
By way of example, in order to produce such a blow-down system, it is possible to employ the arrangements described in applicant's FR-A-92 07903, in which these blow-down means include at least one passage through the tube plate, through which an opening emerges on the upper face of of the tube plate the central region, this opening communicating with drainage means situated outside the casing of the generator.